A multiple hearth furnace comprises an upright cylindrical furnace housing that is divided by a plurality of vertically spaced hearth floors in vertically aligned hearth chambers. A vertical shaft extends centrally through the hearth chambers, passing through each hearth floor. In each hearth chamber at least one rabble arm is fixed to the vertical shaft and extends radially outside there-from over the hearth floor. Such a rabble arm is provided with rabble teeth, which extend down into material being processed on the hearth floor. As the vertical shaft rotates, the rabble arm moves over the material on the respective hearth floor, wherein the rabble teeth plough through the material and mix the latter. Depending on the angle of inclination of the rabble teeth, the material will be moved radially inwardly toward the vertical shaft or outwardly therefrom. Drop holes are provided in each hearth floor, alternately in the inner zone of the hearth floor (i.e. near the vertical shaft) or in the outer zone of the hearth floor (i.e. near the cylindrical furnace housing). Material falling on the inner zone of a hearth floor is moved by the rabble arm radially outwardly over this hearth floor, until it drops through a drop hole in the outer zone of this hearth floor on the outer zone of a hearth floor located directly below. On this lower hearth floor, material is moved by the rabble arm radially inwardly until it drops through a drop hole in the inner zone of this hearth floor on the inner zone of the next lower hearth floor. Thus, material to be processed is caused to move slowly along a serpentine path through the vertically aligned hearth chambers of the furnace.
It is a fact that multiple hearth furnaces possess major advantages over other solid material processing furnaces, such as rotary hearth furnaces, rotary kiln furnaces and shaft furnaces. By allowing a control of different hearth atmospheres and temperatures in the vertically aligned hearth chambers, they allow a very close control of the process inside the furnace. Other advantages of multiple hearth furnaces lie in their ability to maintain the processed materials in mixed condition throughout their passage through the furnace and to warrant a very intense exposure of the solid materials to process gases in a controlled gas/solid material counter flow within the furnace. Nevertheless, since their invention at the end of the nineteenth century, multiple hearth furnaces have only found very few applications in solid material processing. A reason for this lack of confidence in multiple hearth furnaces is that it has never been possible to warrant a problem-free operation of a multiple hearth furnace over longer periods.
The most exposed elements in a multiple hearth furnace are the rabble arms with their rabble teeth. These rabble arms and rabble teeth are subjected to severe temperatures and severe mechanical constraints in a furnace atmosphere that is usually very corrosive. Already in very early multiple hearth furnaces, the rabble arms included a water or gas cooled cast iron support structure, and the rabble teeth were conceived as exchangeable wear parts. Such an exchangeable rabble tooth generally includes a dovetail interlocking element at its upper portion engaging, in a form-fit relationship, a corresponding groove at the underside of the cooled metallic support structure.
An allegedly improved design of a rabble arm was disclosed in 1968 in U.S. Pat. No. 3,419,254. This rabble arm includes a hollow cast iron core obtained by mould casting. It is divided by a central web into two separate passageways for cooling air. The teeth of the arm are formed of a ceramic material. They have an upper fixing portion with a pair of inwardly facing hook-like interlocking elements, which are dimensioned to fit loosely over lower horizontal flanges laterally protruding from the underside of the metallic core. In order to provide an insulating and shock absorbing tight connection between the rabble teeth and the metallic core, a fibrous insulating material is interposed between the hook-like formations and the lower horizontal flanges. To complete the insulation of the metallic core, an inner layer of fibrous insulation is placed over the top part of the metallic core, and an outer solid insulation is finally placed on top of the inner fibrous insulation. Lugs on the metallic core prevent the cover from moving longitudinally with respect to the metallic core. In an alternative embodiment, a plurality of wire-like prongs is welded to the metallic core along its sides and top. Thereafter, a layer of fibrous insulating material is pressed down over the prongs so that it lies snugly over the top of the core. A castable insulation is finally cast over the exterior of the rabble arm, where it is held in place by the wire-like prongs.
A first drawback of known rabble arms is a rather high frequency of teeth breaks in the region of their dovetail or hook-like fixing portion. It will be noted in this context that a break-off of a single tooth may cause severe damages to the rabble arms of the hearth chamber, because the broken off rabble portion is an obstacle for the remaining rabble teeth and may cause a break-off of further teeth or even a collapse of whole rabble arms.
A further drawback of known rabble arms is their insufficient protection against high temperatures. The thermal insulation of known rabble arms is indeed deficient in respect of many aspects. It will be noted e.g. that the underside of the rabble arm, which is exposed to the highest heat load, has the poorest insulation. Furthermore, it happens quite often that the thermal insulation of a rabble arm falls off already after a short operation period of the furnace. As an overhauling of the thermal insulation of a rabble arm requires the removal of the rabble arm, the operator of the furnace usually runs usually the risk not to repair the thermal insulation of the rabble arms until the next major overhauling of the furnace, which requires anyway the dismounting of the rabble arms. In the meantime, the unprotected metallic core of the rabble arm is however exposed to a much higher thermal load than the thermal load it is designed to withstand.
Still another drawback of present rabble arms is a poor wear resistance of their rabble teeth. Indeed, most rabble arms are still equipped with cast iron rabble teeth, which become subject to rapid wear under corrosive hearth atmospheres and/or high hearth temperatures. Ceramic rabble teeth would of course be more wear resistant in such atmospheres, but the manufacture of ceramic form pieces of the size of a rabble tooth is still a rather expensive operation. It follows that the use ceramic rabble teeth is normally economically not justified. Furthermore, ceramic rabble teeth may be very wear resistant but they have nevertheless a low ductility, i.e. they are often subjected to breakage in particular in the region of their dovetail or hook-like fixing portion.
Further rabble tooth structures are disclosed in following documents:
U.S. Pat. No. 1,468,216 discloses a cooled rabble tooth structure comprising a cylindrical hub as fixing portion and a hollow tooth blade as rabble portion. The hollow tooth is integrally cast with the cylindrical hub. The cylindrical hubs are assembled end to end on the elongated metallic support core of the rabble arm and cooperate therewith to direct a cooling medium into the hollow teeth.
DE 389355 discloses a rabble tooth structure comprising a sleeve with a trapezoidal cross-section as fixing portion and at least one rabble blade that is integral with the sleeve and projects from a side wall of the latter. The rabble tooth structure is made of a acid proof refractory material.
U.S. Pat. No. 1,687,935 discloses a rabble tooth structure comprising a dovetail fixing portion engaging a corresponding groove at the underside of the metallic support core of the rabble arm.